A pipe system which is to operate in a borehole usually consists of many lengths or sections of tubing which are assembled end to end to each other. The sections or lengths are typically 2 meters long, and a typical pipe may include 50 or more sections. The pipe will typically also include one or more shorter special sections which, for example, house a well-screen or other sample draw-off port.
The diameter of the pipe depends on the size of the well and the type of measurements to be taken. In a typical case, the diameter of the pipe may be 5 or 6 cm.
It is common for the tube sections of sampling pipes to be made of a non-degradable plastic, such as PVC. When metal is called for (as, for instance, in the said special sections) a stainless steel is preferred, since other metals cannot be relied on not to corrode nor to introduce spurious traces of chemicals which might taint the samples.
One of the requirements of a sampling pipe is that the sections should be quickly and easily assemble-able to each other. Screw threads are common as a mechanical jointing means between sections, and are quite reliable, although expensive to make and time-consuming in use. It has been proposed also to join the plastic pipes together by heat-welding.
One conventional system by which sections of pipe have been joined together may be termed the "tangential-key" system. Here, a key groove is provided between the two sections to be joined together, and a key-hole is provided, through a key may be inserted from outside the tube. The key passes circumferentially around inside the key-groove. A tag of the key may be left protruding from the key-hole, whereby the key can be withdrawn for disassembly.
The tangential-key system provides only axial or lengthwise location of the sections. The tangential-key system does not constrain the sections against relative rotation, but that is usually of little consequence in a sampling pipe.
The invention makes use of the tangential-key system, which is described in more detail below.
In addition to the mechanical jointing system between the sections, it is also usually a requirement, in a sampling pipe, that the sections be sealed together against water leakage and seepage. O-ring seals, residing in seal-grooves, are usually preferred. The seals are intended to prevent water in the well from seeping into the sampling pipe and to prevent liquids, and gases, inside the pipe from leaking out into the groundwater in the well.
One of the tribulations which attends the insertion of pipes into wells and boreholes is the fact that the well is often not straight. It is not uncommon to find that a well may deviate from the truly straight to the extent that adjacent tube sections might be forced to lie at an angle of 1 or 2 degrees, or even more, relative to each other.
(Wells and other boreholes are usually nominally vertical, but sometimes are drilled at other angles.)
Even when the material of the tubes can accommodate distortion somewhat, of course the brunt of the distortion is experienced at the joint.
A characteristic of joint or junction design in previous sampling pipe systems has been that, at the junction, a separate coupling connector has been utilised, being a separate component from the tube sections themselves. Such previous coupling connectors have been of greater diameter than the tubes they are connecting.
It is recognised that the presence of coupling connectors of greater diameter than the sections of pipe is most disadvantageous, especially when the well is not quite straight. It is recognised as much more preferable that the wall thickness of the pipe should be constant along the complete length of the pipe, including the junctions.
The reason such previous coupling connectors have been larger in diameter than the rest of the pipe has usually been that the coupling connector has to house, and provide room for, the O-ring seal.
It is recognised that if the seal layout and arrangement can be such that the O-ring is accommodated within the nominal thickness of the pipe, the components required at the joint for mechanical support and for sealing can be accommodated within the tube sections themselves. It is recognised that when this is the case there then becomes no need, in fact, for the separate coupling connectors to be provided.
The invention is aimed at providing a layout of the junction components which permits the junction not to have an increase in diameter over the rest of the pipe. When that is done, the use of a separate coupling connector then becomes somewhat superfluous. However, separate coupling connectors still may be preferred in some cases, and the invention should not be construed as being limited only to the cases where the tube sections are joined directly to each other.
The invention is concerned with the very tight constraints on the designer regarding the radial dimensions of the components. Wall thicknesses should always be kept as thin as possible. Thus, the designer of sampling pipes and tubes should be constantly seeking to select the slimmest possible O-rings, because these occupy the least amount of radial space.
Nevertheless, the designer wishes to ensure that the O-rings do not leak, and the designer is aware of the usual rule that the slimmer the O-ring, the more tightly the dimensional and other tolerances must be controlled, otherwise the O-ring is likely to break contact with the surface against which it is intended to seal, and thus to leak.
In fact, O-rings are usually provided with sufficient radial squeeze or nip or pinch as is commensurate with the level of hydraulic pressure being supported by the O-ring: an O-ring may leak if the degree of nip is reduced by the out-of-round distortion, even if the seal does not nominally break contact.
Furthermore, in some borehole-sampling situations, it is not enough simply that the seal does not leak. For judicial evidence purposes, for instance, a very high standard of leakage resistance is required. In general, when the pipe is down a well, it is almost impossible to cure a leak; not only that, but special instruments may be required even to detect that the leak is taking place. Furthermore, it is not easy to detect whether the well is in fact straight, nor, if it is not, to determine the angle through which a pipe will be bent. Furthermore again, a pipe junction may leak due to being bent by the borehole, and yet that same pipe junction will display no tendency to leak at the surface, neither before installation into the well nor afterwards when the pipe has been removed.
The usual measure by which a designer copes with O-rings which are prone to leak due to pipe distortion is to provide fatter O-rings, which are more tolerant to distortion. But fat O-rings require thick walls.
Thus, in the case of well-sampling pipes, the problem of leaking seals is an especially demanding one, and at the same time the constraint on the dimensions of the seals is very tight.
The design compromise lies in selecting a seal, whether the usual O-ring or another type of seal, which neither leaks nor occupies much radial space. The invention is aimed at allowing this compromise to be eased, by so arranging the junction that even a slim seal is not prone to leak when the junction distorts.